RFR: 8317975: [JVMCI] assert(pointee != nullptr) failed: invariant

[Doug Simon]

This PR fixes the clearing of a JNI handle help by libjvmci such that it is done at a safepoint. This is achieved by making a VM call to do the clearing.

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Commit messages:
 - clear JNI handles help by libjvmci at a safepoint

Changes: https://git.openjdk.org/jdk/pull/16158/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16158&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317975
  Stats: 31 lines in 5 files changed: 27 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16158.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16158/head:pull/16158

PR: https://git.openjdk.org/jdk/pull/16158